The present invention relates generally to plastic substrates which may be useful in products including, but not limited to, visual display devices, and more particularly to multilayer plastic substrates having improved light transmittance.
As used herein, the term xe2x80x9c(meth)acrylicxe2x80x9d is defined as xe2x80x9cacrylic or methacrylic.xe2x80x9d Also, (meth)acrylate is defined as xe2x80x9cacrylate or methacrylate.xe2x80x9d
As used herein, the term xe2x80x9caverage visible light transmittancexe2x80x9d means the average light transmittance over the visible range from 400 to 800 nm.
As used herein, the term xe2x80x9cpeak visible light transmittancexe2x80x9d means the peak light transmittance over the visible range from 400 to 800 nm.
As used herein, the term xe2x80x9cpolymer precursorxe2x80x9d includes monomers, oligomers, and resins, and combinations thereof. As used herein, the term xe2x80x9cmonomerxe2x80x9d is defined as a molecule of simple structure and low molecular weight that is capable of combining with a number of like or unlike molecules to form a polymer. Examples include, but are not limited to, simple acrylate molecules, for example, hexanedioldiacrylate, or tetraethyleneglycoldiacrylate, styrene, methyl styrene, and combinations thereof. The molecular weight of monomers is generally less than 1000, while for fluorinated monomers, it is generally less than 2000. Monomers may be combined to form oligomers and resins but do not combine to form other monomers.
As used herein, the term xe2x80x9coligomerxe2x80x9d is defined as a compound molecule of at least two monomers that maybe cured by radiation, such as ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermally induced curing. Oligomers include low molecular weight resins. Low molecular weight is defined herein as about 1000 to about 20,000 exclusive of fluorinated monomers. Oligomers are usually liquid or easily liquifiable. Oligomers do not combine to form monomers.
As used herein, the term xe2x80x9cresinxe2x80x9d is defined as a compound having a higher molecular weight (generally greater than 20,000) which is generally solid with no definite melting point. Examples include, but are not limited to, polystyrene resins, epoxy polyamine resins, phenolic resins, and acrylic resins (for example, polymethylmethacrylate), and combinations thereof.
There is a need for versatile visual display devices for electronic products of many different types. Although many current displays use glass substrates, manufacturers have attempted to produce commercial products, primarily liquid crystal display devices, using unbreakable plastic substrates. These attempts have not been completely successful to date because of the quality, temperature, and permeation limitations of polymeric materials. Flexible plastic substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES), have been used in thicknesses from about 0.004 inches to 0.007 inches. However, the surface quality of these substrates is often poor, with the surface having large numbers of scratches, digs, pits, and other defects.
In addition, many polymers exhibit poor oxygen and water vapor permeation resistance, often several orders of magnitude below what is required for product performance. For example, the oxygen transmission rates for materials such polyethylene terephthalate (PET) are as high as 1550 cc/m2/day/micron of thickness (or 8.7 cc/m2/day for 7 mil thickness PET), and the water vapor transmission rates are also in this range. Certain display applications, such as those using organic light emitting devices (OLEDs), require encapsulation that has a maximum oxygen transmission rate of 10xe2x88x924 to 10xe2x88x922 cc/m2/day, and a maximum water vapor transmission rate of 10xe2x88x925 to 10xe2x88x926 g/m2/day.
Barrier coatings have been applied to plastic substrates to decrease their gas and liquid permeability. Barrier coatings typically consist of single layer thin film inorganic materials, such as Al, SiOx, AlOx, and Si3N4 vacuum deposited on polymeric substrates. A single layer coating on PET reduces oxygen permeability to levels of about 0.1 to 1.0 cc/m2/day, and water vapor permeability to about 0.1 to 1.0 g/m2/day. However, those levels are still insufficient for many display devices.
Additionally, many processes used in the manufacture of displays require relatively high temperatures that most polymer substrates cannot tolerate. For example, the recrystallization of amorphous Si to poly-Si in thin film transistors requires substrate temperatures of at least 160xc2x0-250xc2x0 C., even with pulsed excimer laser anneals. The conductivity of a transparent electrode, which is typically made of indium tin oxide (ITO), is greatly improved if deposition occurs above 220xc2x0 C. Polyimide curing generally requires temperatures of 250xc2x0 C. In addition, many of the photolithographic process steps for patterning electrodes are operated in excess of 120xc2x0 C. to enhance processing speeds in the fabrication. These processes are used extensively in the manufacture of display devices, and they have been optimized on glass and silicon substrates. The high temperatures needed for such processes can deform and damage a plastic substrate, and subsequently destroy the display. If displays are to be manufactured on flexible plastic materials, the plastic must be able to withstand the necessary processing conditions, including high temperatures over 100xc2x0 C., harsh chemicals, and mechanical damage.
Thus, there is a need for an improved plastic substrate for visual display devices, and for a method of making such a substrate.
The present invention meets this need by providing a multilayer plastic substrate. The substrate consists essentially of a plurality of thin film layers of at least one polymer, the plurality of thin film layers being adjacent to one another and having sufficient strength to be self-supporting, wherein the multilayer plastic substrate has an average visible light transmittance of greater than about 80%. The average visible light transmittance is typically greater than about 85%, and it can be greater than about 90%. The peak visible transmittance is typically greater than about 85% and it can be greater than about 90%.
There are typically at least about 50 thin film layers. The number of layers depends on the thickness of the thin film layers and the desired overall thickness of the multilayer plastic substrate. The multilayer plastic substrate is typically at least about 0.001 inches thick, and generally at least about 0.004 inches thick. Each thin film layer is typically less than about 50 xcexcm thick.
Polymers include, but are not limited to (meth)acrylate-containing polymers, styrene containing polymers, methyl styrene containing polymers, and fluorinated polymers, and combinations thereof. The glass transition temperature of the at least one polymer is generally greater than about 150xc2x0 C., and it may be greater than about 200xc2x0 C.
The surface roughness of the multilayer plastic substrate is generally less than about 10 nm, and it may be less than about 5 nm, or less than about 2 nm.
The multilayer plastic substrate can have a refractive index of greater than about 1.4 or greater than about 1.5.
The multilayer plastic substrate can include additional layers, including, but not limited to, scratch resistant layers, antireflective coatings, antifingerprint coatings, antistatic coatings, conductive coatings, transparent conductive coatings, and barrier coatings, to provide functionality to the substrate if desired.
Another aspect of the invention involves a method of making the multilayer plastic substrate. The method includes providing a support, depositing a plurality of thin film layers of at least one polymer on the support so that the plurality of thin film layers have sufficient strength to be self-supporting to form the multilayer substrate, and removing the support from the multilayer substrate, wherein the multilayer plastic substrate has an average visible light transmittance of greater than about 80%.
The thin film layers can be deposited in a vacuum. One example of a vacuum deposition process is flash evaporation. In this method, depositing the plurality of thin film layers includes flash evaporating a polymer precursor, condensing the polymer precursor as a liquid film, and cross-linking the polymer precursor to form the polymer. The polymer precursor can be cross-linked by any suitable method, including, but not limited to, radiation curing, such as ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermally induced curing.
Alternatively, the plurality of thin film layers can be deposited by extruding or casting a layer of polymer precursor, and cross-linking the polymer precursor to form the polymer using any suitable cross-linking method.
Accordingly, it is an object of the present invention to provide an improved, multilayer plastic substrate and to provide a method of making such a substrate.